The invention relates to an electrical fuse comprising a closed housing, in which a spring mechanism interrupts an electrical contact as soon as a fuse body serving as a support for the spring mechanism and mainly consisting of an organic material liquefies and relieves the spring mechanism.
Such a fuse is known from U.S. Pat. No. 2,934,628.
Essentially there are two kinds of fuses: those of the first kind serve for current limitation and those of the other kind serve for temperature limitation. In the first kind, a fuse body is heated by the electrical current itself so strongly that it melts at a given maximum permissible current intensity and interrupts the electrical contact in an irreversible manner. However, the invention solely relates to the second kind of fuses, in which the interruption of the contact takes place not by the current, but by external heating at a given maximum permissible temperature. Such fuses are increasingly incorporated in electrical apparatus in order to protect them from overheating and to switch them off in an irreversible manner when a given temperature is exceeded. The switching-off temperature is determined by the melting temperature of the fuse body.
In order that an absolutely reliable operation of the fuses can be guaranteed even for long operating times, the following particular requirements have to be imposed on the fuse body as the actual switching element:
Suitable materials must have a melting point associated with the relevant switching-off temperature. In order that the fuses act rapidly, they should operate as far as possible at an accurately defined melting point and not in a melting range. Therefore, the waxes mentioned in U.S. Pat. No. 2,934,628 are not suitable as material for the fuse body.
Upon melting the fuse bodies have to flow out as as easily and rapidly as possible in order to enable the springs within the fuse to open the contact.
During the whole operating time, the fuse bodies are not only subjected to a given spring stress, but are also subjected to a thermal variation load. They have to be capable of withstanding the spring force at temperatures which from time to time lie just below the relevant melting point. Of course the shape of the fuse bodies (cylinder, hollow cylinder, sphere etc.) and the manufacturing method strongly influence the pressure strength that can be attained. At any rate it has to be ensured that the fuse bodies are capable of withstanding a multiple of the spring stress occuring during operation and do not undergo critical length or shape variations during the required operating time.
For a reliable operation of the fuse bodies, during the manufacture different tolerance limits have to be taken into account. Consequently, materials should not be used which upon heating cause irregular or irreversible length variations and which exhibit, for example, below the operating temperature, a phase variation and a volume variation connected therewith (for example the .alpha.KNO.sub.3 .fwdarw..beta.KNO.sub.3, phase variation at 129.degree. C.).
With respect to the corrosion within the fuses, it should be ensured that the materials used for the fuse bodies are not allowed at any rate to attack the surfaces of the contacts or of the housing. Since the fuses are generally sealed in a vacuum-tight manner during the manufacture, with the use of hygroscopic materials no problems are to be expected during operation, because moisture can be kept remote from the fuses. However, additional measures can become necessary for the production process.
In the relevant fields of use, temperatures may be reached at which already clearly observable vapour pressures of the material of the fuse bodies can occur. This especially applies to most of the alkaline materials. Due to inhomogeneous temperatures of the fuse, mass transports of fuse body materials through the gaseous phase can thus take place and the operation of the electrical contacts can be adversely affected. Therefore, a comparatively low vapour pressure at operating temperatures is of major importance when choosing the materials for the fuse body.
Finally, the materials for the fuse bodies have to be available in the largest possible quantities and at the lowest possible cost and it has to be possible to process them readily by casting or moulding to form stable mouldings.